A drawback of lean premixed, low emission combustion systems is that they exhibit an increased risk in generating thermo-acoustically induced combustion oscillations. Such oscillations, which have been a well-known problem since the early days of gas turbine development, are due to the strong coupling between fluctuations of heat release rate and pressure and can cause mechanical and thermal damage and limit the operating regime.
A possibility to suppress such oscillations consists in attaching damping devices, such as quarter wave tubes, Helmholtz dampers or acoustic screens.
A reheat combustion system for a gas turbine including an acoustic screen is described in patent application DE 103 25 691. The acoustic screen, which is provided inside the mixing tube or combustion chamber, consists of two perforated walls. The volume between both walls can be seen as multiple integrated Helmholtz volumes. The backward perforated plate allows an impingement cooling of the plate facing the hot combustion chamber.
However, it is a drawback of this solution that an impingement cooling mass flow is required to prevent hot gases to enter from the combustion chamber into the damping volume. This massflow, however, decreases the damping efficiency. If the impingement mass flow is too small, the hot gases recirculate passing through the adjacent holes of the acoustic screen. This phenomenon is known as hot gas ingestion. In case of hot gas ingestion the temperature rises in the damping volume. This leads to an increase of the speed of sound and finally to a shift of the frequency, for which the damping system has been designed.
The frequency shift can lead to a strong decrease in damping efficiency. In addition, as the hot gas recirculates in the damping volume, the cooling efficiency is decreased, which can lead to thermal damage of the damping device. Moreover, using a high cooling mass flow increases the amount of air, which does not take place in the combustion. This results in a higher firing temperature and thus leads to an increase of the NOx emissions.
A solution for avoiding some of the mentioned issues is described, for example, in patent application EP 2 295 864. This document discloses a combustion device for a gas turbine, wherein a multitude of layers are braced together to form single compact Helmholtz dampers, which are cooled using an internal near-wall cooling technique close to the hot combustion chamber. Therefore, the cooling mass flow can be drastically reduced without facing the problem of hot gas ingestion, leading to less emissions and a higher damping efficiency. As single Helmholtz dampers are used, different frequencies can be addressed separately. Whether single nor a cluster of Helmholtz dampers are used, the design is based on an appropriate implementation of a near wall cooling.
A multitude of near wall cooling patents can be found, see e.g. a perforated laminated material (U.S. Pat. No. 4,168,348), a cooled blade for a gas turbine (US 2001 016 162) or a cooled wall part (DE 44 43 864). Especially the object of U.S. Pat. No. 4,168,348 is closely linked to the device according to EP 2 295 864 as it is built up using several plates laminated together to obtain the complex cooling channels.
Published European patent application EP 2 362 147 describes various solutions on how the near-wall cooling can be realized. The near-wall cooling passages are either straight passages or they show coil shaped structures parallel to the laminated plates. A drawback of this solution is that measures have to be implemented to establish a symmetric velocity profile at the opening towards the acoustic damping volume. The near wall cooling passage has to be designed in such a way that the flow field inside the acoustic neck is not influenced by the cooling mass flow entering the acoustic damping volume.
Measures to realize an adequate velocity inlet profile at the openings towards the acoustic damping volume are described in patent application EP 2 299 177. To avoid the above-mentioned impact, always a pair of cooling channels enters the damping volume at the same location in opposite direction. Of Course, multiple pairs of cooling channels can also enter the damping volume at the same location. To reduce the kinetic energy of the flow and to restrict a possible fluctuating motion of the cooling air inside the opposite channels, the channels are separated using a barrier. In addition the end of the cooling passage is designed in form of a diffuser to reduce the velocity of the cooling mass flow in front of the barrier. The additional measures to realize an adequate velocity inlet profile increase the design efforts and react sensitive to the common manufacturing tolerances.
A potential problem in operation of such “near wall cooling” or “micro cooling” systems is the risk of debris. The cooling air from the compressor of a gas turbine plant may contain dust particles that tend to block the flow of air through the micro cooling channels. But due to the above-mentioned reasons and due to a negative influence on the efficiency of the gas turbine larger dimensioned cooling channels (with the consequence of an increased flow of cooling air) are not applicable.